A composite picture taken from security camera and released by the Thai Royal Police showing a suspect who Thai police believes to be involved with the bomb attack walks with backpack (L) and leaves the Erawan Shirne (R) shortly before bomb exploding outside the Erawan Shrine in Bangkok, Thailand, August 17, 2015.[Photo/IC]

BANGKOK - Thai police said Tuesday investigators believe a man seen in security video wearing a yellow T-shirt and carrying a backpack set off the explosion at a central Bangkok shrine that killed 22 people and injured more than 100.

"The yellow shirt guy is not just the suspect. He is the bomber," Police Lt. Gen. Prawut Thavornsiri, a police spokesman, told The Associated Press.

The man suspected of the bombing at the Erawan shrine was seen in grainy CCTV footage entering the compound with a backpack on, sitting down against a railing and then slipping out of the bag's straps.

Wearing a yellow shirt and with shaggy, dark hair, the young man then stands up and walks out holding a blue plastic bag and what appears to be a mobile phone. The backpack was left by the fence as tourists milled about.

National police chief Somyot Pumpanmuang said the suspect could be Thai or foreign.

"That man was carrying a backpack and walked past the scene at the time of the incident. But we need to look at the before and after CCTV footage to see if there is a link," Somyot told a news conference.

Police earlier said they had not ruled out any group, including elements opposed to the military government, for the bombing at the shrine, although officials said the attack did not match the tactics of Muslim insurgents in the south.

Prime Minister Prayuth Chan-ocha called Monday's explosion at a busy intersection "the worst incident that has ever happened in Thailand," and he promised to track down those responsible.

"There have been minor bombs or just noise, but this time they aimed for innocent lives," Prayuth said. "They want to destroy our economy, our tourism."

Without elaborating about possible perpetrators, the prime minister said Tuesday, "Today we have seen the closed-circuit footage, we saw some suspects, but it wasn't clear. We have to find them first."

People light candles at the Erawan shrine, the site of Monday's deadly blast, in central Bangkok, Thailand, August 18, 2015. [Photo/Agencies]